You werent perfectly clear.... are we talking about a bow with glass front and back, or an all wooden bow... say, just three lams of wood?
If we are talking about an all wooden tri-lam, I prefer to taper the middle piece(and often the backing) and leave the belly piece parallel. This allows more material available for tillering and weight reduction without overly thinning the belly lam, which can bring you dangerously close to, or through, the glue line... which can hold potentially serious consequences, especially if different woods are used for the core and belly, with the belly wood better suited to resisting compression forces.
Degree of taper and distance between the fadeouts... I vary them according to other design parameters... like width and side profiles, bow length, handle shape, distance from handle to flares, limb cross section... etc. So it's hard to say without knowing more about your bow. Mine have finished as short between the fades as 9" and as long as 16".
In my opinion, some folks make their fadeouts too short and steep right off the bat... at glueup. I'd rather see them extend a good 2" or more past the flares with a more gradual slope... and then they can be finessed/tillered to help gradually slow the flexing into the handle area later during tillering. But again, much depends on design, limbs and handle shape, and needs considered and coordinated.
What type of woods will you be using?